The present invention relates to multiple-layered pipe, preferably of two or more thermoplastic resins, and preferably having three layers comprising at least one outer layer, an intermediate core, and at least one inner layer, and a process and apparatus for the manufacture of such pipe. The present invention is particularly useful for producing pipe having an intermediate core of expanded thermoplastic resin.
Many attempts have been made to produce a multiple layered pipe in order to combine the desirable characteristics of different thermoplastic resins. These attempts have usually involved extruding several streams of thermoplastic resins through a series of concentric tubes which are fixed relative to one another in a radial manner, such as by spiders, to define annular passages therebetween, and then subsequently joining the resultant annular layers to produce a multiple-layered pipe. Pipes prepared by such processes and apparatus, however, have had spider marks thereon due to the flow interruptions caused by the many spiders necessitated in such apparatus. Moreover, such apparatus contain adjusting screws which protrude into the individual die passages for adjusting the thickness of the annular layers. These screws also interfere with the flow of the molten resin as it is being extruded, whereby additional marks have been developed upon the pipe.
Furthermore, it is undesirable to provide an extrusion die for each layer of the desired pipe. Since high internal pressure is required for the extrusion of the highly viscous heat plastified thermoplastic material, such apparatus are subject to distortions in the die which cause undesirable nonuniformity in the thickness of each layer, unless the extrusion pressures of each layer are balanced. However, in practice, it is frequently difficult to maintain extrusion pressures constant in their proper relationship. Those types of apparatus employing a separate die for each layer thus inherently involve difficulty in controlling the thickness of each layer in the desired pipe.
Another disadvantage associated with prior art apparatus for the production of co-extruded plastic pipe is the inability of the prior art apparatus to equalize the flow of thermoplastic within the die so that the heat plastified thermoplastic is applied as a continuous layer of constant thickness. This problem is particularly prevalent in those apparatus in which layers of thermoplastic are extruded through radial orifices. In order to form a layer of constant thickness, it is absolutely essential in such apparatus that the flow of thermoplastic through the orifice is constant along its entire circumference.
In the extrusion of multiple-layered pipe having an intermediate core of expanded thermoplastic material, this inability to form uniform layers of thermoplastic becomes even more objectionable. In order to produce a thermoplastic pipe with an expanded core which possesses high mechanical strength, it is absolutely necessary that each layer has a constant thickness. Failure to achieve very precise uniformity of thickness of each layer in expanded pipe, results in a product which has inferior physical properties.
In U.S. Pat. No. 3,223,761 is disclosed a process and apparatus for producing multiple-layered plastic tubing without a separate die for each layer of tubing. Several streams of the desired thermoplastic are fed to a multi-ported laydown which produces a composite multiple-layered cylindrical stream. This stream is then transported to an annular die having a mandrel which forms the cylindrical stream into an annular stream. However, in order to maintain layer uniformity of thickness, it is necessary that the extrusion process occur under objectionally high pressure conditions and with a velocity below the threshold of turbulent flow to preserve laminar flow conditions. Such apparatus and process are limited, however, to the extrusion of a three-layered pipe of only two different materials. For the extrusion of pipe containing more than two thermoplastic resins, a plurality of extruders having tapered nozzles are selectively positioned within the cylindrical stream to form a core of a new thermoplastic material. These extruders interfere with the melt flow, yielding a pipe with manufacturing imperfections. To maintain uniformity of layer thickness in this prior art apparatus, it is furthermore essential that the very precise process conditions described above be maintained, a difficult fact to accomplish in practice.
U.S. Pat. Nos. 3,447,204, 3,819,792, and 4,061,461 disclose die apparatus for producing a multiple-layered tube without employing the die-within-the-die concept. Such apparatus is limited, however, to the production of only two layered structures. Moreover, with such apparatus the flow of thermoplastic through the extrusion passageways is not balanced, precluding thereby the formation of layers of uniform thickness.
As has been stated above, the disadvantages associated with the above processes and apparatus are particularly acute in the production of multiple-layered thermoplastic pipe with an intermediate core of expanded thermoplastic material. The smaller amount of expensive thermoplastic resin in the core region, and the decrease in weight associated therewith, has made expanded pipe very attractive. Accordingly, it was proposed in U.S. Pat. No. 3,782,870 to produce a thermoplastic multiple-layered pipe with an intermediate core of expanded material. Such prior art expanded pipe has been marked, however, by prohibitatively low mechanical strength, low resistance to shock, high brittleness, and generally to be of inferior quality to non-expanded thermoplastic pipe. Moreover, such pipe has been manufactured from only a single thermoplastic resin and thus possesses inferior characteristics when compared to non-expanded pipe combining the desirable properties of several thermoplastic resins.
One attempt to produce a multiple-layered pipe having an intermediate expanded core without a separate die for each layer is disclosed in U.S. Pat. No. 3,299,192 in which a multiple-layered tube is formed by extruding a stream of thermoplastic containing a blowing agent, converting the stream into an annular stream, and then forming inner and outer skins thereon by quench chilling the annular stream. Necessarily, such a method is limited to the manufacture of pipe from only a single thermoplastic resin. Pipe produced by this method, therefore, exhibits the inferior physical characteristics associated with expanded pipe manufactured from only a single thermoplastic material.
The above-noted physical defects associated with prior art expanded pipe can be attributed to a failure in the prior art to develop a process and apparatus which can effectively control to very precise tolerances the uniformity of thickness of each layer. The inability of the prior art to control precisely layer thickness can be further attributed to the inability to produce plastic flow equalization throughout the narrow flow passages within a die. The inability of the prior art to produce a practicable expanded pipe is indicative, more generally, of the inability of the prior art to produce multiple-layered pipe of at least two thermoplastic resins with a uniform and desired thickness for each layer.
It would be desirable, therefore, if an apparatus and process for the production of multiple-layered pipe were available which could attain very precise control of layer thickness and flow equalization of the thermoplastic throughout the narrow extrusion passages of the die.